Scattering over the displaced surface is not supported yet. If that feature is important to you, you can cast your vote here: https://forum.corona-renderer.com/ The most wanted Chaos Scatter feature? the option is called “render-time displacement support”.
For the moment the workaround is to convert displacement to geometry and use it for scattering. You can optimize/approximate converted geometry to reduce polygon count and make it invisible for the render. Displaced surface would be used for visual representation and optimized geometry for scattering. I hope that makes sense.